In recent years, the needs to lower the grain size of metallic powders used in electronic parts are increasing with the advance of miniaturization of the electronic parts used in portable terminals, etc. Nickel ultra-fine powder used for laminated ceramic capacitors are exemplary. As a process for producing nickel ultra-fine powder, there is a method including the step of heating and evaporating a chloride raw material having a high vapor pressure within a CVD apparatus, and further supplying hydrogen gas as a reducing agent into the apparatus to reduce the raw material, thereby producing metallic nickel ultra-fine powder having a grain size of not larger than 1 μm.
Such a production method is recognized to be suitable for producing fine powder, because in the method the raw material is evaporated to be reduced and deposited at a relatively low temperature of approximately 1000° C. (i.e. not higher than the melting point of nickel). However, since such a method uses a CVD apparatus, it necessitates expensive electrical energy to heat the raw material, and since the method uses expensive hydrogen as a reductive gas, it is costly. Moreover, since the method is reductive reaction of a chloride by hydrogen, there is a problem that it necessitates costly equipment in which the possibility of corrosion, leaks and the like of the apparatus is properly addressed, because poisonous gases such as chloride gas and hydrogen chloride gas are generated in a furnace, as is disclosed in Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. H04-365806).
On the other hand, there is a method for producing iron fine powder which includes burning a hydrogen-containing fuel with an oxygen-containing gas using a burner, and supplying vaporized iron chloride into the flame to cause high-temperature hydrolysis, thereby producing iron fine powder, as is disclosed in Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. Showa 56-149330). This method is relatively low cost, because it does not use costly electrical energy to control the atmosphere of the reductive reaction field, and further it necessitates no hydrogen gas. However, similarly to the above method, it necessitates countermeasures against generation of chloride gas and hydrogen chloride, because the method uses chloride as a raw material. Moreover, the metallic powders which are produced have a grain size which diverges widely from 40 to 80 μm, and hence there is a problem in the controllability of grain size. Moreover, there is a problem in that it is not suitable for producing ultra-fine powder having a grain size of not higher than 1.0 μm, which meets the needs people have these days.
With all of the above-mentioned conventional technology, it is necessary to use a chloride having a high vapor pressure for producing metallic powder particularly fine powder, and hence the form of raw material is restricted.
The present invention was made in view of the above circumstances, and it is an object of the present invention to provide a process for producing metallic ultra-fine powder, which makes it possible to use a raw material which is spread over a wide range, freely control the grain size of the metallic powder to be produced, and reduce the grain size to be not higher than 1/10 of that of the raw material in the case in which the raw material is a powdery one, at low cost and with high safety.